In the early 1990s civilian (non-NSA) cryptanalysts developed a "new" and very powerful technique called "differential cryptanalysis", that basically allows you to guess the key of DES-like ciphers by encrypting different (known) cleartexts and observing the ciphertext that comes out. By comparing the different inputs and outputs, a reasonable guess can be made as to the key. But guess what: DES was found to be very resistant to this attack. Why? It was discovered that the exact nature of the S-boxes (the "substitution boxes" or "tables of values", as you refer to them) were formulated in such a way as to thwart the technique. What does this tell us? Firstly, that the NSA had already developed differential cryptanalysis (or an analogue thereof) in the mid 1970s, a full two decades ahead of its rediscovery by civilians. Secondly, and this is the clincher, it tells us that the NSA didn't weaken DES: it strengthened it immesurably. Talk about irony.